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Hellenistic science, Greek astronomy

Page history last edited by Marek 12 years, 7 months ago


The Lyceum after Aristotle

Aristotle met Theophrastus on the island of Lesbos (ca. 371-ca. 286 BC), and Theophrastus returned to Athens with him in 335 BC. Theophrastus participated in the Lyceum for 13 years until the Aristotle's death in 322 BC, at which point he took the reins for the next 36 years. Theophrastus wasn't afraid to disagree with some of the teachings of his mentor; in fact, he disagreed with Aristotle's:

     1. teoleology- Theophrastus believed that not everthing has purpose

     2. 4 elements theory- Theophrastus questioned Aristotle's status of fire as an element

     3. theory of light and vision- questioned Aristotle's opinion that light is the actualization of the transparency of the medium

     When Theophrastus died, he left the library of his and Aristotle's books to Neleus, whom he may have intended to succeed him. However, Strato was elected to be the new leader of the Lyceum, at which point Neleus took the books home with him to Skepsis in Asia Minor. The books were eventually purchased from Neleus's ancestors and remained in Athens until 86 BC(E) when the Romans, under the command of general Lucius Cornelius Sulla, swept through, taking the books with them. The Romans, then, edited and circulated them. Strato led the Lyceum from 286-268 BC(E); his material ideas included calling into question the theory of 4 elements. Strato also demonstrated acceleration and discredited Aristotle's theory of motion.




     Early Greek astronomy was considered mathematical rather than physical and largely based on Babylonian obeservations. The early Greeks were primarly concerned with observing and mapping stars and plotting solar and lunar motions to create a satisfactory calendar. It was important that it predicted when the seasons changed, and that a proper balance between the solar year and lunar months be struck. To have knowledge of the beginning of summer and winter, it was necessary to know when the soltices would occur, in the case of spring and autumn, the equinoxes were key. There was no concern for the causes of motions observed in the heavens.  Not until the 4th century B.C. did major changes occur under the prodding of Plato (427-348/47 BC) and Eudoxus (ca. 390-ca. 337 BC). For the philosophers, determined to show order in the universe, the motions of Mercury, Venus, Mars, Jupiter and Saturn with their apparent irregularities (stopping, moving backward, changing latitude) became the focus of discussion.  Greek astronomers made attempts to explain how it was possible for the universe to be an orderly place, and still have these apparent irregularities.


The Basics


     The first general model for the universe was a two-sphere model. The two concentric spheres were Earth and the celestial sphere, on which were placed the stars, and on which the planets moved in their appropriate circles. The celestial sphere rotated once daily explaining the daily movement of the heavenly bodies. The equator of the Earth projected onto the celestial sphere was called the celestial equator. On this outer sphere the Sun traced a circle in its yearly motion through the Zodiak known as the ecliptic. It was known to be tilted by about 23 degrees with respect to the celestial equator. The equator and the ecliptic intersected at two points on the celestial sphere - the equinoxes, which determine the beginning of spring and autumn. The two points on the sphere at which the ecliptic and equator are farthest appart were called the soltices, and these were the beginning of winter and summer. Circles drawn parallel to the plane of the equator through the soltices were called the Tropics of Cancer (through the summer solstice) and Capricorn (through the winter solstice). The Tropics represent the smallest circles of the spiral the Sun makes (combination of daily and yearly motion) in its journey around the Earth. The Greek astronomers were also familiar with the varying speed of the Sun on the ecliptic, the stops and reversals in the motions of the planets (called retograde motion) and the oddity of Mercury and Venus seamingly tied to the position of the Sun.


Eudoxus and the 4-sphere model


     To this somewhat crude model, the first important additions were made by Eudoxus. Eudoxus' model envisioned four concentric spheres for each planet to explain each of its constituent motions. The outermost sphere was responsible for the daily motion east to west. The next sphere, with an axis tilted 23 degrees with respect to the first, produced the planet's motion along the ecliptic, and the last two accounted for the irregularities. The innermost sphere was also the one to which the planet was attached. The Sun and the Moon only required three spheres, since they do not exhibit retrograde motion. The details of Eudoxus' system are unknown, but it can be shown that a combination of the movements of the two innermost spheres may produce a hippopede - a very crude model of the retrograde motion. What is important about this model is that it is the first serious attempt at a geometric model explaining the motion of the planets. Eudoxus was looking to find mathematical order hidden in the apparent irregularities and he did. His model provides rough agreement with observation and it is conceivable that this was the goal all along. The model was not meant to be quantitatively predictive or have any physical interpretation, although Aristotle attempted to give it one. Aristotle's physical model had 55 spheres total and a mechanism of passing on the diurnal motion from the celestial sphere, while counteracting any specific movements of planets.

A Moving Earth


     The geocentric model with a fixed Earth was not the only one considered by the Greeks. Heraclides (ca. 390- 339 BC) for example, proposed that the Earth rotated on its axis once a day thus explaining the rising and setting of the celestial bodies. Two generations after Heraclides, Aristarchus analyzed the size and positions of the Sun and Moon to justify a heliocentric model of the universe. The two models: geocentric and heliocentric, were in that time mathematically equivalent, but the geocentric model was preferred, and the heliocentric largely dismissed. There were many important reasons for this. Common sense would have one believe that the Earth is fixed, while the stars and planets move. It was also against religious belief to say otherwise. Most profoundly, however, Aristotelian physics gave an explanation for the movement of the heavens. Any model in which the Earth moved caused an upheaval. As long as Aristotelian physics were not challenged, it was impossible to concider a motion in the heavens that was not a uniform circular motion.


Hipparchus and the Star Catalogue


     Another major influence of the time was Hipparchus (2nd century BC). All we have are just pieces of his works and an impressive and extensive catalogue of the stars according to their brightness and positions in the sky. Hipparchus had access to centuries of Babylonian observations and mappings of stars and planets. This immense amount of empirical data allowed him to calculate the procession of the equinoxes and the length of the lunar month to within one second of the modern value. He created a superior star map by use of a new sighting instrument called the diopter. All this lead to an appreciation of exact quantitative prediction, which Cladius Ptolemy would attempt with his astronomical model 300 years later.


Differences Between Physical and Mathematical Astronomy


There are many differences between the fields of physical and mathematical astronomy even though they deal with the same subject matter. The mathematical astronomer does not deal with the causes of what he observes. His goal is to create a geometrical model which saves as much of the observed motions as possible and has most predictive power. What motivates his choices is expediency. The mathematical model that is chosen is the simplist one of those available. The physical astronomer on the other hand is most concerned with cause and substance. He must search for the method behind the motion. He cannot escape the constraints of reality.



Primary Sources


  "... the astronomer, when he proves facts from external conditions, is not qualified to judge of the cause, as when, for instance, he declares the earth of the stars to be spherical; sometimes he does not even desire to ascertain the cause, as when he discourses about an eclipse; at other times he invents by way of hypothesis, and states certain expedients by the assumption of which the phenomena will be saved. For example, why do the sun, the moon, and the planets appear to move irregularly?"

- Simplicius 'Commentary on Aristotle's Physics'

This segment of Simplicius explains the lack of concern of the astronomer about whether his hypotheses reflect reality. There is no requirement for a cause in astronomy.



Key Terms and Definitions


retrograde motion: the backward motion of the planets as seen against the fixed stars

physical astronomy: requires causation in the astronomical models

mathematical astronomy: interested only in predictions

equinox: point of intersection of the celestial equator with the ecliptic (there are two)

procession of the equinoxes: the motion of the equinoxes as observed against the fixed stars; one complete revolution requires 23000 years

ecliptic: circle on the celestial sphere, tilted 23 degrees w.r.t. the equator, passes through the center of the Zodiak

solstices: points on the celestial sphere at which the ecliptic is farthest from the equator

star catalog: the extensive catalog of stars recorded by Hipparchus that catagorized stars by their brightness and their positions in the sky





Relevant Links


(This is a link to the diagrams Dr. Ramberg used in class. Just click on the corresponding links to get more information)



Comments (16)

Alyson Collins said

at 6:28 pm on Sep 9, 2008

Hey all, I'm finished with the summary, have at it. I must say I had difficulty keeping it to 100 words! I stiil went over slightly, oh well. I wasn't really sure what Dr. Ramberg meant by intergrating an idea not covered in the lecture. I put in a blip about astronomy before the 4th century, but anything else seemed to be to long. If any of you think this isn't enough for the 'other idea', let me know and I'll try to find someting else.

Grant Berry said

at 8:33 pm on Sep 9, 2008

I added information on Eudoxus' 4 sphere model. I had written down in my notes today that the eccentric and epicycle on deferent model were from Apollonius, but in the book these are attributed to Ptolemy. According to Lindberg, Apollonius was known for his work on conic sections, not on astronomy. That's why I stopped with the information on "Apollonius." If it truly is something from Ptolemy, it will have to wait.

Did anyone else have this discrepancy?

liz mastroianni said

at 9:52 pm on Sep 9, 2008

i just added some information that i thought would help better explain some of the ideas

jgm829@... said

at 10:30 am on Sep 10, 2008

I added some info on the origins of Greek astronomy.

Nicole Hagstrom said

at 3:38 pm on Sep 10, 2008

I made some grammatical changes, mostly eliminating repetitious phrases, and added in a slight amount of information on Heraclides and Aristarchus.

Grant Berry said

at 7:40 pm on Sep 10, 2008

Just fixed some spelling.

Mark Philippi said

at 9:34 pm on Sep 10, 2008

I added the part about the Lyceum after Aristotle. I didn't exactly know where to fit it into what was already there, but it might be better placed later.

Garrett McCormack said

at 10:28 pm on Sep 10, 2008

i added some definitions and fixed a few things

Kristy Carey said

at 9:26 am on Sep 11, 2008

I did some grammatical and basic editing for clarity and brevity.

Garry Polley said

at 11:06 am on Sep 11, 2008

First off... I do agree with grant that some of the lecture was switched or messed up. But anyway. I created and added Eudoxus' model of the figure eight idea. I understand it better if I can reference the picture.

Garry Polley said

at 11:07 am on Sep 11, 2008

I noticed I made two errors... but not at the same time so I had to edit both at separate times.

Keriann Collins said

at 11:51 am on Sep 11, 2008

I have to go to class; I changed some of the formatting and made a comment on the mathematical nature of early Greek astronomy. However, there are some problems with the summary that still need revising. It says that Greek astronomy developed from Babylonian astronomy in the Hellenistic period, but then that Plato was one of the two that brought change to it in the fourth cent. BC(E). But Plato is pre-Hellenistic.

Douglas Elliott said

at 1:03 pm on Sep 11, 2008

Well written. I added more detail to the discussion of the roman sacking of the lyceum in the first century BCE.

Alyson Collins said

at 1:18 pm on Sep 11, 2008

hey, all looked it over pretty good I see the discrepencies Keri pointed it out, was trying to starighten them out but ran out of time. I put birth/death dates of the astronomers in too help clarify but I lost my changes. I'll go through it again sometime after class

Alyson Collins said

at 2:25 pm on Sep 13, 2008

As promised I added birth/death dates. I also tried to clarify the different layers of ancient Grk astronomy. Remeber that the ppl in the lecture were over an expanse of time. Also I looked in wikipedia to try to clarify the layers to myself and it helped (though I know its not always reliable). It kinda divided Greek astronomy into archaic (the kind only interested in stars/calendars), then Eudoxean (or something like that) astronomy w/ Eudoxus and Plato, then Hellenistic astronomy. Though our astronomers might of overlapped some of the categories, I thought it helped clarify how Plato was a contemporary of Eudoxus and was pre- hellenistic at the same time. I think it's because Eudoxus was pretty much pre-Hellenistic too. Hope that helped some of you

Marek said

at 7:12 pm on Sep 18, 2008

Fixed all the errors I think, also removed the few sentences about Apollonius of Perga, since he isn't important in the development of Astronomy. The idea of epicycles existed before him, and they will be best discussed along with Ptolemy I believe.

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